Removal of vaporized cooling liquid from heat exchange element by power jets



. HRUDA 3,414,753

REMOVAL OF VAPORIZED COOLING LIQUID FROM HEAT EXCHANGE ELEMENT BY POWER JETS FIG. I.

Filed Dec.

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Jxy fi 705 j 1 J 1.1 l 68 72-U 74 76 WITNESSES: INVENTOR 204 W Robert M. Hrudu ATTORNEY United States Patent REMOVAL OF VAPORIZED COOLING LIQUID FROM HEAT EXCHANGE ELEMENT BY POWER JETS Robert M. Hrnda, Horseheads, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 1, 1964, Ser. No. 415,097 2 Claims. (Cl. 313-35) This invention relates generally to improved electron discharge devices and in particular to those devices utilizing a vapor phase cooling system to remove the excess heat emitted by the elements of this device. More specifically, this invention relates to those electron devices whereby the cooling of the anode element is effected by the vaporization of a cooling liquid, usually Water, in which the anode element is immersed.

Broadly speaking, there are three conventional cooling systems which are utilized to remove the heat generated in high power electron discharge devices. First, it is well known in the art to force a stream of air past the external surface or fin of the element (usually an anode element) of the electron discharge device to absorb and dissipate the heat. Because air is a relatively poor heat conductor, such air cooled systems have been limited to lower levels of heat dissipation. It has been estimated that forced air systems are capable of removing only about 50 watts per square centimeter of effective internal anode element area. Further in order to dissipate higher dissipation levels, such systems require large, expensive blowers to properly circulate the air.

A second method which has been employed in high power electron discharge devices uses circulating water as a heat transfer medium. Typically, circulating water can remove approximately 100 watts per square meter of effective internal anode element area or approximately twice the anode element dissipation rating of its air cooled counterpart. The principal drawback of such systems is that the water temperature must be maintained substantially below 100 C. to prevent vapor being formed on the element to be cooled. If vapor does form on the element, the cooling medium will in effect be insulated by the vapor from the element causing localized hot spots and possibly destroying the electron discharge device. In practice, the temperature of water leaving the electron discharge device is limited to 70 C. to preclude the possibility of vaporization.

The most efficient method for removal of excess heat utilizes a vapor phase cooling system in which the dissipation of heat is effected by the vaporization of the cooling medium upon the surface of the element to be cooled. The particular advantage of a vapor phase cooling system is that great amounts of heat may be absorbed from an element of the electron discharge device by converting the cooling medium in which the element is immersed into vapor. Therefore, the efficiency of a vapor phase cooling system is dependent upon the heat of vaporization of the cooling medium whereas the efficiency of a water cooled system is dependent upon the specific heat of the cooling medium. For example, in those systems utilizing water as a cooling medium, raising the temperature of one gram of water from 40 C. to 70 C. would absorb only 30 calories of energy, whereas transforming one gram of water at 100 C. to steam vapor would absorb 540 calories. Therefore in comparison, a vapor phase cooling system with a given amount of water may absorb twenty times the amount of heat as a water-cooling system.

In a typical vapor phase cooling system, an element usually the anode element is immersed in a cooling medium such as water. As the electron discharge device is 3,414,753 Patented Dec. 3, 1968 placed into operation, the water held by a boiler in contact with the anode element is vaporized. thereby absorbing a great amount of heat. The vapor is directed to a condenser where the water gives up its: thermal energy and is converted back to the liquid state. This condensate is then returned to the boiler, completing the cycle. A distinguishing feature of such a vapor phase cooling system is the configuration of the anode element. In practice, the anode element is made of a highly heat conductive material such as copper and has a great number of teeth which allows the cooling medium such as water to be vaporized without producing the damaging hot spots upon the anode element. The teeth of the anode element effect a twofold increase of the heat dissipation from this element: (1) a greatly increased anode surf-ace area is produced through which the heat may be dissipated, and (2) each of the teeth provides a conductive path through which the thermal energy may be more efficiently transmitted into the cooling medium.

Even though the vapor phase cooling system described above provides a significant improvement over either a forced air cooling system or a circulating water system, such vapor cooling systems are capable of only removing approximately watts per square centimeter of the effective external anode element area before significant damage will be caused to the anode element. As will be explained in greater detail later, this limitation is primarily due to the formation of a layer of vapor about the teeth of the anode element. The formation of this vapor layer commonly identified with calefaction effectively insulates the anode surface from the cooling medium and prevents the discharging or bubbling of the vapor from the anode element. As the process of calefaction proceeds, the temperature of the anode element will rapidly rise to a point where the material of the anode element begins to melt and the anode element as well as the other elements of the device will be destroyed.

Accordingly, it is the general object of this invention to provide an improved and more efficient electron discharge device.

It is a further object of this invention to provide a more efiicient system of cooling an electron discharge device.

It is another object of this invention to provide an improved vapor phase cooling system for an electron discharge device.

It is a more particular object of this invention to provide an improved vapor phase cooling system wherein the rate of heat dissipation exceeds 135 watts per square centimeter of the effective external anode element area.

It is another object of this invention to provide an improved vapor phase cooling system for an electron discharge device wherein the amount and rate of flow of the cooling medium may be significantly reduced.

A still further object of this invention is to provide a vapor cooling system wherein the use of a steam manifold and condenser may be eliminated.

Briefly, the present invention accomplishes the above cited objects by effectively removing the layer of vapor formed on the teeth of the anode element of a vapor phase cooling system. By removing the insulating vapor layer, the vapor phase cooling system is substantially prevented from entering the calefaction region of operation where the anode element will be destroyed by overheating. In one particular embodiment of the invention, the cooling medium, usually water, is forced through a plurality of pipes or risers to remove the vapor layer with jets of the cooling medium. The effect of removing the vapor level is twofold: first, the vapor layer is effectively broken up and the resulting bubbles are swept away from the hotter portions of the surface of the anode element, and secondly, the bubbles of vapor are: immediately condensed by the enveloping cooling medium. The heat may then be removed from the cooling medium and recirculated or the cooling medium may be simply discarded.

Further objects and advantages of the invention will become more apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 shows in a sectioned view a vapor phase cooling system in which an embodiment of this invention has been incorporated;

FIG. 2 shows a partial, cross sectional view of the vapor phase system of FIG. 1 taken along line IIII of FIG. 1;

FIG. 3 shows an enlarged, detailed view of the pipes or risers incorporated in the vapor phase cooling system shown in both FIGS. 1 and 2;

FIG. 4 shows in schematic form a preferred embodiment of the vapor phase cooling system of this invention; and

FIG. 5 shows a graph depicting the heat transfer capability, measured in watts per square centimeter of a surface of an anode element of this invention in contact with water at various temperature gradients.

Referring now to FIG. 1, there is shown an electron discharge device of the vapor phase variety. The electron discharge device 10 comprises an envelope 12 made of a ceramic or glass material and a plurality of terminals 14 extending therethrough to provide electrical connection with the internal elements (not shown) of this device. The internal elements of the electron discharge device 10 are contained within an evacuated chamber formed by the envelope 12 and an anode or target element 16 is axially aligned of the envelope 12. The anode element 16 has a plurality of wedge shaped teeth 18 which are disposed about the periphery of the anode element 16. Further, the anode element 16 is immersed within a cooling medium 22 which is held within a container or boiler jacket 20. The boiler jacket has a bottom plate 24 through which are inserted an inlet conduit 32 and an outlet conduit 34. A manifold 26 is disposed within the boiler jacket 20 above the inlet conduit 32 to receive the cooling medium 22 as it is forced through the inlet conduit 32. A plurality of tube like structures or risers 28 are disposed in a circular configuration with each of the risers 28 positioned between two of the teeth 18. Further, each of the risers 28 is connected through a manifold opening 30 to the manifold 26 to allow the cooling medium 22 to be circulated therethrough and to be directed onto the anode element 16 through the coolant directing nozzles or apertures formed in the inner walls of the risers 28. A support plate 36 upon which the anode element 16 and the envelope 12 are mounted is secured to an annular rim 37 of the boiler jacket 20 by such means as bolts 38.

It is noted that the selection of the particular material out of which the various elements of this device is constructed is not believed to be critical. However, it is desired to used the same material to construct those elements in contact with the cooling medium 22 to prevent a possible electrolytic action within the boiler jacket 20. In practice, a high thermally conductive material such as copper has been used to make the anode element 16, the risers 28 and the boiler jacket 20. Further, a purified and deionized water is used as the cooling medium 22 to prevent possible electrolysis and the formation of sludge on the elements of this device.

Referring now to FIG. 2, a plan view of the anode element 16 and the risers 28 is presented to show the precise placement of the risers 28 and how the cooling medium 22 is forced through the apertures 40 onto the anode element 16. The anode element 16 has a plurality of teeth 18 extending radially outward toward the boiler jacket 20. A root portion or surface 42 of the anode element 16 is formed between the teeth 18 and is that portion of the anode element 16 upon hich the cooling medium 22 is directed. The root portion 42 is that portion of the anode surface where the maximum heat stress occurs. Further, the teeth 18 have lateral portions 44 extending along the surface thereof from the root portions 42 radially outward to a tip portion 46 facing the boiler jacket 20. As will be explained in greater detail later a vapor layer 48 (shown in dotted line) tends to form on the root portion 42 and the lateral portion 44 of the anode element 16 and it is necessary to direct the cooling medium 22 through the apertures 40 to break up or disperse the vapor layer 48 and to drive the bubbles of vapor into a condensation region 50.

Referrin now to FIG. 3, a single riser 23 is shown which comprises a cylindrical, tube like structurewith a plurality of evenly spaced apertures 40 therein. In order to build up the desired pressure of the cooling medium, an end cap 54 is placed over the end of the riser 28 opposie the manifold 26. Further, a curved connecting portion 52 attaches the riser 28 to the manifold 26. As may be easily understood the pressure with which the cooling medium 22 may be directed onto the anode element 16 is dependent upon the diameter of the riser 28 and the diameter of the apertures 40. In one exemplary embodiment, the cooling medium 22 is maintained at a pressure of 10.5 pounds per square inch by making the outer diameter of the riser 28, .5 inch, and providing ten apertures 40 being spaced from each other by .4 inch and having a diameter of .046 inch.

Referring now to FIG. 4, a preferred embodiment of a complete vapor phase cooling system is shown. As previously described, such a system includes the electron discharge device 10 having an envelope 12 and a boiler jacket 20. As will be explained later in greater detail, a cooling medium is directed into the boiler jacket 20 through the inlet conduit 32 and onto the anode element 16 through the risers 28. The cooling medium 22 having absorbed the heat generated by the anode element 16 is circulated out of the boiler jacket 20 through the outlet conduit 34, an insulation conduit 66 and a conduit 68 to a heat exchanger 70. A recirculation pump 66 is interconnected through a conduit '76 to the heat exchanger 70 and is further connected to the boiler jacket 20 a conduit 64, an insulation conduit 62 and the inlet conduit 32. In an illustrative system, the recirculation pump 66 was designed to have a capacity of circulating 22.5 gallons per minute at a pressure drop of 20 p.s.i. across the pump. Since the boiler jacket 20 is often maintained at the high potential of the anode element 16, it is necessary to insulate the electron discharge device from the remaining portions of the vapor phase cooling system; therefore, as shown in FIG. 4, the insulation conduits 60 and 62 are inserted to electrically isolate the heat exchanger 70, the recirculation pump 66 and their associated conduits. Further, a second cooling medium such as water may be circulated through an inlet 72 and an outlet 74 of the heat exchanger 70 to dissipate the heat absorbed by the cooling medium 22.

In operation, the cooling medium 22 is directed into the boiler jacket through the inlet conduit 32; the cooling medium 22 is then dispersed in the manifold 26 to each of the plurality of risers 28. As best shown in FIG. 2, the cooling medium 22 is forced through the plurality of apertures 40 onto the root portion 42 of the anode element 16. As will be explained in greater detail, the principal limitation of those vapor phase cooling systems or vaportrons known in the art is caused by the formation of the vapor layer 48 upon the surfaces of the anode element 16. As the vapor layer 48 coats the entire outer surface of the anode element 16, calefaction will occur causing the temperature of the anode element 16 to rapidly rise to a point at which the anode element 16 begins to melt and is subsequently destroyed. A significant contribution of this invention lies in the means to prevent formation of the vapor layer 48. In the embodiment of the invention shown in FIGS. 1 and 2, the cooling liquid 22 is forced under pressure through apertures onto the root portion 42 of the anode element 16 thereby disrupting the vapor layer 48 that tends to form along the root portions 42 and the lateral portions 48. Thus, the vapor layer 48 is broken up into an eruption of bubbles which flow outward under the pressure of the cooling medium 22 along the lateral portion 44 into the region 50 where the bubbles are condensed. The cooling medium 22 having absorbed the heat generated by the anode ele ment 16 is circulated out of the boiler jacket 20 through the outlet conduit 34 and through the insulation conduit and conduit 68 to the heat exchanger 70. The recirculation pump 66 forces the cooling medium 22 through conduit 68 to the heat exchanger 70 where the absorbed thermal energy is dissipated by the second cooling medium that is circulated through the inlet and outlet conduits 72 and 74. The cooling medium 22 after dissipating its absorbed thermal energy in the heat exchanger 70 is returned through conduits 76 and 64, and insulation conduit 62 into the boiler jacket 20, thus completing thecycle.

In order to properly appreciate the contribution of this invention, an explanation of the physical laws of heat transfer through boiling water will be made with regard to those vapor phase cooling systems known in the art and the invention disclosed herein. Heat transfer capability (measured in watts per square centimeter) of a heated surface, in contact with water at various temperature gradients (measured in degrees centigrade), is plotted in the Nukiyama curve shown in FIG. 5. The first portion of the curve, i.e., from point B to point C, indicates that up to about 108 0., heat transfer is a linear function of the temperature differential between the surface of the heat radiating element and the water, reaching a maximum of about 5 watts per square centimeter at point C. In this so-called convection cooling region, boiling takes place in the heated water at a distance from the surface of the radiating element. From point C at 108 C. to point D at 125 C. heat transfer increases as the fourth power of the temperature dicerence until, at point D, it reaches 135 watts per square centimeter. In this region, the hubbles of vapor are formed at the surface of the radiating element, and break away to the surrounding cooling medium. The action of the heat transfer in this region is termed nucleate boiling. Above point D an unstable portion of the Nukiyama curve is seen, where increasing the temperature of the heated surface actually reduces the unit thermal conductivity. As the temperature is increased from point D to point B at 225 C., a vapor layer is formed on the heat radiating surface thereby insulating the surface from the water. At point E called the Leidenfrost point, the heat radiating surface becomes completely covered with a layer of vapor and all heat transfer is conducted through this layer. From point E to point F, the film vaporization zone, the heat transfer increases with temperature until at about 1000 C., the value of 135 watts per square centimeter is again reached.

Though it may not be entirely evident from the logarithmic graph of FIG. 5, the region from B to D is a relatively narrow area in comparison with the region from D to F; further, if a heat radiating surface is subjected to unlimited heat, the heat radiating surface will quickly pass from point D to point F. This irreversible super-heating is known as calefaction. For the anode element of an electron discharge device the passing into total calefaction could not be tolerated, as any unit heat transfer density above 135 watts per square centimeter would result in temperatures above 1000 C. at which point the anode element (as well as the other elements of the device) would easily be destroyed.

The operation of vapor phase cooling systems is dependent upon maintaining the heat transfer characteristic of at least a portion of the surface of the anode element at a temperature below point D on the Nukiyama curve.

Therefore in order to increase the heat dissipating capacity, the anode elements of vapor phase cooling systems have been provided with a plurality of thick, vertical fins on the exterior surface of the element. Thus, the anode fins or teeth provide an additional external, radiating surface and also provide heat conducting paths radially extending away from the anode element.

Referring specifically to FIGS. 2 and 5, when operating in the region between B and D, boiling takes place in the root portions 42 of the anode element 16. Increasing the thermal energy dissipated by the anode element 16 causes the boiling area to move outward over the lateral portion 44 toward the tip portion 46. Though in the operation of anode elements of the prior art it has been possible to obtain temperatures up to about 180 C. at the root portion of the fin or teeth, it has been necessary to maintain the tip portion of the tooth at a temperature less than C. If the tip portion is allowed to exceed 125 C., a vapor layer will form over the entire surface of the anode element and thus effectively insulate the surface of the anode element from the cooling medium and cause the anode surface to pass rapidly into the calefaction region thereby destroying the anode element.

However, in accordance with the teachings of this invention, the vapor layer 48 which tends to form on the surface of the anode elements 16 is immediately broken up by the flow of the cooling medium 22 through the apertures 40. As the jets of the cooling medium 22 are directed upon the root portion 42, the vapor layer 48 is disrupted and is caused to form a stream of bubbles which are directed outwardly along the lateral portion 44 into the region 50 where the vapor is condensed. The significant result of disrupting the vapor layer is that the anode element 16 may be heated to a point significantly above 125 and thereby substantially avoiding the operation of the anode element in the calefaction region. As shown in the graph of FIG. 5 and as explained above, the anode elements of known vapor phase cooling systems must be substantially operated in the region between C and D to prevent the destruction of the anode element (though it has been theorized that a portion of the anode element may be operated in the region from D to E). Further, it may be seen from this graph that the vapor phase cooling systems of the prior art are limited with respect to their heat transfer capacity to approximately watts per square centimeter; the graph of FIG. 5 shows that the heat transfer characteristic of a radiating surface will only exceed 135 Watts per square centimeter at temperatures of approximately 1000 C. However, if the vapor layer may be substantially disrupted as taught by this invention, the surface of the anode element may operate in a region of the curve from D to G which appears to be an extension of the nucleate boiling region C to D. At present, the outermost limit of this new region D to G is not known; however, it appears that the point G on the graph is substantially above that point at which the anode element would be otherwise destroyed. Therefore as a result of the teachings of this invention, the surface of the anode element may be operated at temperatures significantly above 125 C. and exhibit a heat transfer capability substantially in excess of 135 watts per square centimeter, the practical limit of the heat transfer characteristic of the anode elements of known vapor phase cooling systems.

It will, therefore, be apparent that there has been disclosed an improved vapor phase cooling system which is capable of more efficiently dissipating the thermal energy generated by the anode element. More specifically, by removing the vapor layer which tends to coat the surface of the anode element by directing a plurality of jets of the cooling medium onto the anode element surface, the heat transfer capability of the external surface may be increased above the present limit of 135 watts per square centimeter. At present, it is believed that the cooling system in accordance with this invention does operate in an extended region of the nucleate boiling zone and at temperatures significantly above 125 C. Those vapor phase cooling systems known in the art have achieved a maximum heat dissipation from a typical electron discharge device of approximately 45,000 watts, whereas a vapor phase cooling system incorporating the teachings of this invention has been applied to the same device to achieve a heat dissipation in excess of 75,000 Watts Without reaching a maximum operating condition.

While there have been shOWn and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A method of cooling a heat exchange member having a plurality of fins extending therefrom, said method comprising the steps of immersing said heat exchange member into a cooling liquid, supplying sufiicient thermal energy to said heat exchange member so that the vapor of said cooling liquid is formed on the surface of said heat exchange member, and directing said cooling liquid onto said heat exchange member with sufiicient force to break up and remove the vapor of said cooling liquid from said heat exchange member.

2. The method of cooling an anode element of an electron discharge device having a plurality of fins extending therefrom, said method comprising the steps of immersing said anode element into a cooling liquid, supplying sufficient thermal energy to said anode element so that the vapor of said cooling liquid is formed on the surface of said anode element, and directing jets of said cooling liquid between said fins of said anode element onto the surface of said anode element to break up and remove the vapor of said cooling liquid from said anode element.

References Cited UNITED STATES PATENTS 2,302,513 11/1942 Abraham 165l 2,772,540 12/1956 Vierkotter l65l 10 X 2,873,954 2/1959 Protze 31324 3,034,769 5/1962 Bertin et a1 165174 3,235,004 2/1966 Beurtheret 165-185 FOREIGN PATENTS 631,197 12/1961 Italy.

ROBERT SEGAL, Primary Examiner. 

1. A METHOD OF COOLING A HEAT EXCHANGE MEMBER HAVING A PLURALITY OF FINS EXTENDING THEREFROM, SAID METHOD COMPRISING THE STEPS OF IMMERSING SAID HEAT EXCHANGE MEMBER INTO A COOLING LIQUID, SUPPLYING SUFFICIENT THERMAL ENERGY TO SAID HEAT EXCHANGE MEMBER SO THAT THE VAPOR OF SAID COOLING LIQUID IS FORMED ON THE SURFACE OF SAID HEAT EXCHANGE MEMBER, AND DIRECTING SAID COOLING LIQUID ONTO SAID HEAT EXCHANGE MEMBER. 